This invention relates to an efficient green-emitting luminescent material and to a method for production thereof. More particularly, it relates to such luminescent material which can be exited by infrared stimulation to produce green visible radiation.
So-called infrared radiation-to-visible radiation converting phosphors are excited at two or three stages by two or three photons of infrared radiation, and emit one photon of visible light.
Luminescent materials of high conversion efficiency have recently been developed, and research efforts on them have advanced to such an extent that they can be put into practical use as visible light-emitting diodes in combination with an infrared-emitting diode such as gallium arsenic diode, In addition, such infrared-to-visible converting luminescent materials are expected to be applied for infrared detection as in infrared Lasers, or be utilized as indicating screens excitable with infrared radiation or as Lasers or light sources excitable with infrared radiation.
Typical instances of known green-emitting, infrared-to-visible converting phosphors are a product having a composition expressed by LnF.sub.3 : Yb.sup.3.sup.+,Er.sup.3.sup.+ or LnF.sub.3 :Yb.sup.3.sup.+, Ho.sup.3.sup.+ (wherein Ln is at least one element selected from La, Y, Gd and Lu), as taught in French Pat. No. 2,004,990, and a product having a composition expressed by BaMF.sub.5 :Yb.sup.3.sup.+,Er.sup.3.sup.+ or BaMF.sub.5 :Yb.sup.3.sup.+,Ho.sup.3.sup.+ (wherein M is at least one element selected from the group consisting of Y, Gd, Ce, Sc, La, Ga, In and Al), as illustrated in West German Laid-Open Specification No. 2,018,352.
Known green-emitting phosphors which are inferior to the above-exemplified ones with respect to brightness of emission, include LiYF.sub.4 :Yb.sup.3.sup.+,Ho.sup.3.sup.+ (R. K. Watts, J. Chem. Phys., 53 (9) 3552 (1970)), Na.sub.0.5 Y.sub.0.5 WO.sub.4 :Yb.sup.3.sup.+,Er.sup.3.sup.+, (J. P. van der Ziel, L. G. Van Uitert, and W. H. Grodkiewicz, Appl. Phys., 41 (8), 3308 (1970)), etc. Even YF.sub.3 :Yb.sup.3.sup.+,Er.sup.3.sup.+ or LaF.sub.3 :Yb.sup.3.sup.+,Er.sup.3.sup.+, which is known to be a luminescent material producing brightest emission among these known phosphors, is inferior to a GaP:N green emitting diode belonging to the competitive art, with respect to conversion efficiency, when it is pumped by an infrared-emitting diode. Accordingly, development of a novel luminescent material having a higher conversion efficiency has been demanded in the art for broad utilization of light-emitting elements using an infrared-to-visible converting luminescent material.